During the 150-day experiment, the mulch biofilm barrier
showed a 97.2–99% removal efficiency for phenanthrene and 99.9%
for pyrene. Sorption and biodegradation of PAHs gave stable
operation to the system for the phenanthrene and pyrene mixture
in aqueous solution. For abiotic control columns, after 32 days of
sodium azide injection, the effluent concentration of phenanthrene
began to increase slowly. Surfactant could significantly increase the
solubility of phenanthrene. However, for surfactant solubilizedhenanthrene and pyrene, the biobarrier showed an 80–98%
removal efficiency for surfactant solubilized phenanthrene, and
28–91% removal for pyrene during the initial 25 days of surfactant
injection. After that, the biobarrier seemed to lose its sorption
capacity, and overall a 60–83% removal efficiency was obtained for
phenanthrene, and an 8–30% removal efficiency for pyrene was
achieved. The sorption capacity of the mulch prevented fast
migration of hydrocarbons and helped the biobarrier to overcome
its initial lag phase of bacterial biofilm formation. However, for high
molecular weight pyrene in the presence of surfactant, the bio-
barrier showed onlya small removal rateafter the sorption capacity
was gone because of the recalcitrance of pyrene. For high molecular
weight PAHs, a sorption material with an increased sorption
capacity might be required to prevent the migration of surfactant
solubilized PAH. For that, addition of an alternative supporting
material such as activated carbon with mulch might be helpful to
prevent the migration of high molecular weight PAH. A bacterial
culture which is able to degrade high molecular weight PAHs could
also be inoculated with surfactant injection in order to increase the
bioavailability of the high molecular weight PAH (
Boochn et al.,
1998; Thibault et al.,1996
). In addition to these, pulsed injection of
surfactant might give more room for bacteria to remove the
surfactant solubilized PAHs in the biobarrier than with continuous
surfactant injection over a long period